Some implantable medical devices use magnets to hold internal and external pieces in proper position. For example, as shown in FIG. 1, an idealized cochlear implant system may include a receiving coil 108 located under the skin 103 and embedded in or just on top of the bone 104. A receiver magnet 106 is contained in the center of the receiving coil 108. An external transmitter housing 101 includes a transmitter magnet 105 that is positioned over the receiver magnet 106 so that the external transmitter housing 101 is held in place in an optimum position adjacent to the receiving coil assembly 102. When such an optimal position is maintained, an external transmitting coil 107 within the transmitter housing 101 can use inductive coupling to transmit a transcutaneous data and/or power signal to the receiving coil 108.
The receiving coil 108 may, for example, be encapsulated within some tissue-compatible organic material such as silicone or epoxy, forming a receiving coil assembly 102. In such an arrangement, the receiver coil assembly 102 is connected to receiver electronic circuits within a metal or ceramic case which is hermetically sealed from the surrounding tissue. Or, in another approach, the receiver magnet 106, receiving coil 108 and the receiver electronic circuits are all contained within a common hermetic case. In any such arrangement, the receiver magnet 106 is a permanently integrated part of the implant structure.
One problem arises when the patient undergoes Magnetic Resonance Imaging (MRI) examination. Interactions occur between the receiver magnet and the applied external magnetic field for the MRI. As shown in FIG. 2, the external magnetic field  from the MRI may create a torque  on an implanted receiver magnet 202, which may displace the receiver magnet 202 or the whole coil assembly 201 out of proper position. Among other things, this may damage the adjacent tissue in the patient. In addition, the external magnetic field  from the MRI may reduce or remove the magnetization of the receiver magnet 202. As a result, the demagnetized receiver magnet 202 may no longer be strong enough after exposure to the external magnetic field  of the MRI to hold the external transmitter housing in proper position. The implanted receiver magnet 202 may also cause imaging artifacts in the MRI image, there may be induced voltages in the receiving coil, and hearing artifacts due to the interaction of the external magnetic field  of the MRI with the implanted device.
Therefore, implants with removable magnets have been developed. FIG. 3 shows a portion of a typical implant system using magnets according to one approach used in the prior art. An external transmitter housing 301 includes transmitting coils 302 and an external holding magnet 303. The external holding magnet 303 has a conventional coin-shape and north and south magnetic poles as shown which produce external magnetic field lines 304. Implanted under the patient's skin is a corresponding receiver assembly 305 having similar receiving coils 306 and an internal holding magnet 307. The internal holding magnet 307 also has a coin-shape and north and south magnetic poles as shown which produce internal magnetic field lines 308. The internal receiver housing 305 is surgically implanted and fixed in place within the patient's body. The external transmitter housing 301 is placed in proper position over the skin covering the internal receiver assembly 305 and held in place by interaction between the internal magnetic field lines 308 and the external magnetic field lines 304. Rf signals from the transmitter coils 302 couple data and/or power to the receiving coil 306 which is in communication with an implanted processor module (not shown).
The arrangement in FIG. 3 differs from the earlier prior art in that the implant is designed so that the internal holding magnet 307 is removable by a first pre-MRI surgery. This eliminates the problems of torque, demagnetization, and image artifacts caused by the magnet during the MRI procedure. Then, after the MRI, a second post-MRI surgery is necessary to replace the internal holding magnetic 307. While this arrangement allows implant users to receive MRI's when necessary, the requirement for two surgeries raises issues and problems of its own.
More recently, some MRI related problems have been addressed by using an implanted magnet structured to avoid producing torque in an MRI field. One example of such an arrangement is shown in FIG. 4, which is based on the disclosure of U.S. Patent Publication 20060244560, the contents of which are incorporated herein by reference. The external transmitter housing 401 is basically the same as in FIG. 3, with transmitting coils 402 and an external holding magnet 403. The implanted receiver assembly 404 has corresponding receiving coils 405 and an internal holding magnet 406, as well as connecting wiring 407 to a separate processor module. But in FIG. 4, the internal holding magnet 406 has a cylindrical or spherical shape. A ball-shaped welded case 408 (e.g., of titanium or niobium) hermetically encapsulates and isolates the internal holding magnet 406 from the body tissues (otherwise, it might rapidly corrode).
As a result, the internal holding magnet 406 is able to rotate to re-align itself to an external MRI magnetic field without producing a torque, becoming demagnetized, or creating induced voltages, etc. This avoids many of the problems of their earlier arrangement shown in FIG. 3. Typically, a patient having an implant as shown in FIG. 4 may undergo MRI without surgeries to remove and replace the internal holding magnet 406. But even in this arrangement, there may still be imaging artifacts due to the internal holding magnet 406, especially in the nearby region adjacent to the magnet.